Ti2AlNb Alloy Research Articles

Investigation of the O phase in the Ti–22Al–25Nb alloy during deformation at elevated temperatures: Plastic deformation mechanism and effect on B2 grain boundary embrittlement

The Ti–22Al–25Nb alloy, which possesses a complex microstructure, is a new type of lightweight high-temperature structural material. Revealing the effects of microstructure and temperature on the mechanical properties of this alloy is crucial for applications in the aerospace industry. In this study, we designed three types of microstructures of the Ti–22Al–25Nb alloy, namely, single-phase B2, dual-phase α2 + B2, and triple-phase O + α2 + B2, and investigated their deformation and fracture behavior with increasing temperature (25,750,and 930 °C). The O + α2 + B2 sample possessed the best comprehensive mechanical properties, and the softening of the O phase at elevated temperatures was mainly due to massive {001}<110>O basal slip and twinning. The evolution of O phase twins with increasing temperature can be described as follows: (021) → (021) + (110) → (110). At 750 °C, (021) twins formed in the strip-shaped O phase in B2 grains, while (110) twins formed in the spheroidized O phase at B2 grain boundaries. In addition, B2 grain boundary embrittlement was observed at elevated temperatures, and its mechanism can be summarized as follows. Elevated temperatures induced element segregation at the grain boundaries between hypersaturated-state B2 grains, enriching Ti and Al and diffusing Nb into the B2 grains on the nanometer scale, leading to a thin-shell O phase with a thickness of tens of nanometers precipitating at the B2 grain boundaries. During elevated-temperature fracturing, cracks propagated quickly along the interface between the thin-shell O phase and B2 phase, leading to embrittlement of the B2 grain boundary. The precipitation of many fine granular O or α2 phases at the B2 grain boundaries can inhibit the precipitation of the thin-shell O phase and hinder the propagation of cracks along the grain boundaries. Therefore, the microstructure of this alloy should be controlled to precipitate many fine granular O or α2 phases to occupy the B2 grain boundaries and inhibit its elevated-temperature embrittlement.

Microstructure and Mechanical Properties of Electron Beam Welded Joints of Ti2AlNb Alloy

Microstructure and Mechanical Properties of Electron Beam Welded Joints of Ti2AlNb Alloy

Microstructural characterization and mechanical properties of Ti3Al-Nb/Ni-based superalloy laser-welded joints with different type of V/Cu composite interlayers

Dissimilar welding of Ti3Al-Nb (Ti-24Al-17Nb) alloy and Ni-based superalloy with different type of V/Cu composite interlayers was performed by laser welding. Detailed microstructural characterization and mechanical properties of the laser-welded joints were investigated. The results indicated that laser welding can be achieved by using the three types of different V/Cu composite interlayers (Type I-0.2 mmV/0.2 mmCu, Type II-0.2 mmV/0.4 mmCu and Type III-0.2 mmV/0.6 mmCu). However, the type of V/Cu composite interlayers has a great influence on the composition distribution, microstructure and mechanical properties of the joints. When Type I was used, the dissimilar weld zone displayed mainly equiaxed and dendrite with fine grain size, and micro-crack was found in the weld. When Type II was used, the weld zone mixed of fine grain region (V-rich phase and Cu-rich phase) and coarse grain region (dendrite V-based solid solution and granular Cu-based solid solution in two-phase separation state). When Type III was used, the weld zone was mainly composed of large Cu solid solution, spherical Ni2V, Ni2V3 and unmelted V layer. The maximum room/high (873 K) temperature tensile strength of the joints was 312 MPa/240 MPa, which was obtained by using Type II. All the joints fractured at the interface on Ti3Al-Nb alloy side, indicating that the interface zone intermetallics were the main factor of affecting the mechanical properties of the joints. Moreover, the interface reaction layer between Ti3Al-Nb and unmetled V layer was also a weak link of Type III joint.

Thermal Stability of Ti6242S and Ti–6Al–7Nb Alloys

Thermal Stability of Ti6242S and Ti–6Al–7Nb Alloys

Microstructural stability and creep properties of middle Nb γ-TiAl alloy with a modulated microstructure

In this work, a modulated structure composed of nano-scale Ti2AlNb (O phase) and α2 phase was obtained in middle Nb γ-TiAl alloy. The O phase nucleates at the α2/γ interface and is orthogonal to the α2/γ interface. The creep testing results show that the modulated structure exhibited the better creep performance compared with the fully lamellar microstructure, manifesting in a smaller minimum creep rate and creep strain after creep for 200 h. The O phase in α2 lamellae suppressed the α2 →γ phase transformation and constructed the coherent interface with adjacent γ lamellae, which replaced part of the semi coherent α2/γ interface, to reduce the generation of interfacial dislocation loops, thereby improving the creep resistance. Besides, discontinuous precipitations (DPs) caused by (α2/γ) → β0(ω0) + γ phase transformation were observed in the crept samples, contributing to the microstructure degradation. The present work shows significant engineering significance and scientific value for improving the creep performance and microstructure stability for middle Nb–TiAl alloys.

Oxidative Corrosion Mechanism of Ti2AlNb-Based Alloys during Alternate High Temperature-Salt Spray Exposure

This study investigates the corrosion damage mechanisms of Ti2AlNb-based alloys under high temperature, salt spray and coupled high temperature-salt spray conditions. This alloy was analysed in detail from macroscopic to microscopic by means of microscale detection (XRD, SEM and EDS). The results indicated that Ti2AlNb-based alloy surface oxide layer is dense and complete, and the thickness is only 3 µm after oxidation at 650 °C for 400 h. Compared to the original sample, the production of the passivation film resulted in almost no damage to Ti2AlNb-based alloy after 50 cycles of salt spray testing at room temperature. The tests showed that Ti2AlNb alloy shows good erosion resistance at 650 °C and in salt spray. However, this alloy had an oxide layer thickness of up to 30 µm and obvious corrosion pits on the surface after 50 cycles of corrosion under alternating high temperature-salt spray conditions. The Cl2 produced by the mixed salt eutectic reaction acted as a catalytic carrier to accelerate the volatilisation of the chloride inside the oxide layer and the re-oxidation of the substrate. In addition, the growth of unprotected corrosion products (Na2TiO3, NaNbO3 and AlNbO4) altered the internal structure of the oxide layer, destroying the surface densification and causing severe damage to the alloy surface.

Effect of Preheating on the Microstructure Evolution and Mechanical Properties of Electron Beam Welded Ti2AlNb Alloy

Effect of Preheating on the Microstructure Evolution and Mechanical Properties of Electron Beam Welded Ti2AlNb Alloy

Additively manufactured novel Ti6Al7Nb circular honeycomb cellular solid for energy absorbing applications

This investigation addresses a novel additively manufactured circular honeycomb sample of the titanium alloy Ti6Al7Nb which exhibits superior mechanical properties compared to hexagonal honeycomb samples. Compared to other lattice structures/cellular solids of Ti-based alloys, the circular honeycomb sample provides an optimum combination of strength and fracture strain. During compression testing, the circular honeycomb exhibits a wide stress-strain curve with higher tensile stress and elongation compared to hexagonal honeycomb samples. The failure of the circular honeycomb sample occurs layer-by-layer, so the whole sample fails not before the fracture of all single layers of the cells in the circular honeycomb leading to higher elongations. In this regard, the hexagonal honeycomb sample fails by shear of struts oriented at 45° to the loading axis. The area near the fracture region shows a fine acicular αʹ microstructure. Electron Backscattered Diffraction (EBSD) data analysis reveals coarse prior β grains in the surrounding microstructure. So, the circular honeycomb sample is superior to the hexagonal honeycomb sample concerning impact and energy absorption applications.

Microstructure evolution and mechanical properties of a high-strength Ti2AlNb alloy processed by large-strain two-pass hot extrusion

The microstructure evolution and tensile behavior of Ti2AlNb alloy (Ti-22Al-23Nb-1Mo-1Zr) processed by two-pass hot extrusion and two-step heat treatment were investigated. After extruding deformation in the B2 phase field, the B2 grains were refined from about 1500–50 µm through dynamic recrystallization. The microstructures subjected to solution treatment in the B2 + α2 phase field and aging treatment in the B2 + O phase field include B2 matrix, lamellar O, rim O and lamellarα2/O phases. The microstructure and tensile properties are sensitive to aging temperature. As the increase of the aging temperature, the content of the B2 phase and the size of the lamellar O phase increase, resulting in low strength and high ductility. The alloy aged at 800 °C has a good match of strength and ductility. It has an elongation of 6.3% with a tensile strength as high as 1225 MPa due to the precipitation strengthening of numerous lamellar O phase and fine grain strengthening. The tensile fracture mode predominantly shows quasi-cleavage characteristics with typical cleavage facets and massive dimples, which is consistent with the good balance of tensile strength and ductility. An in-depth understanding of the relationship between microstructure and tensile properties in this study can be helpful for the engineering application of the Ti2AlNb alloy.

Fretting wear behaviors and mechanism of a high Nb–TiAl alloy with full lamellar microstructure at ambient temperature

Fretting wear behaviors and mechanism of a high Nb–TiAl alloy with full lamellar microstructure at ambient temperature

Formation of columnar strengthening phases and mechanical properties of Nb–Ti–Al alloy by spark plasma sintering

In this study, Nb-27.3Ti–18Al (at.%) alloys with columnar strengthening phases were fabricated by combining spark plasma sintering (SPS) at 1250 °C and 1350 °C and mechanical alloying (MA). Two morphological powders were obtained by the MA process. Powder I was the completely alloyed powder with homogeneous Nb-based solid solution phase (Nbss), and Powder II consisted of elemental lamellae formed by cold welding. Microstructure evolution at different SPS temperatures of two kinds of powders and toughening mechanisms of alloys were analyzed, and the results have shown that Powder II could promote the formation of columnar phase besides sintering temperature. Moreover, after comparison and analysis, grain refinement of SPSed alloy with 27.3 at.% Ti content showed good fracture toughness at 1350 °C, which is better than the previously reported Nb–Ti–Al alloys by hot pressing (HP) and laser forming.

A concept of regulating the distribution of continuous brittle phases by spatial division of the Ti3SiC2/Ti2AlNb brazing seam

Herein, a novel concept was proposed to address the issue that continuous brittle phases degrade the mechanical properties of a brazed joint. In this concept, the continuous brittle phases could become dispersive by inserting a network-like structure to divide the brazing seam spatially. This concept was verified in a specific brazing system, where Ti3SiC2 ceramic and Ti2AlNb alloy were brazed by an AgCu interlayer. Ductile Cu foam with a network-like structure was used to disperse the continuous brittle TiCuSi intermetallic that should have in-situ formed in the Ti3SiC2/AgCu/Ti2AlNb brazed joint. Results show that the effect of Cu foam on dispersing the brittle TiCuSi intermetallic is pronounced. The effects of holding time and pore size on the dispersion effect of Cu foam were investigated. The joint fracture analysis and micro-mechanical properties of phases analysis reveal that the dispersive TiCuSi intermetallic is superior to the continuous one. The reason is that the microstructure of discrete TiCuSi intermetallic surrounded by ductile Ag-based and Cu-based solid solutions has a more robust capacity to accommodate microcracks. Thus the brazed joint is toughened and strengthened.

Microstructure characteristics of the directionally solidified TiAl-Nb alloys with different solidification modes

Microstructure characteristics are investigated for the TiAl-Nb alloys with four typical solidification modes, namely, single β, hypo-peritectic, hyper-peritectic and single α solidification. The fully lamellar structures is formed in both the dendrite and interdendritic regions in single β, hypo-peritectic, and hyper-peritectic solidification alloys, and the order of lamellar thickness is hyper-peritectic, hypo-peritectic and single β solidification alloys. Some γ islands originating from the prior interdendritic regions are formed by the chemical segregation in hyper-peritectic solidification alloy, but in single α solidification alloy, some fine lamellar structure only exist in the dendritic core and the granular γ grains are observed in the primary α interdendritic region. For hypo-peritectic solidification alloy, lamellar grain boundaries are almost located in the core of primary β phases. Lamellar orientation can be changed along either the boundaries of dendrites or the ridge-like Nb-segregations. However, for hyper-peritectic solidification alloy, the lamellar boundary locates in both the interdendritic region and the core of primary β dendrite. The lamellar orientation is changed mainly along the dendritic Nb-segregation, not along the dendrite boundary. Not every α2 in lamellar grain has Burgers orientation relationship with the prior β dendrite for both hypo- and hyper-peritectic solidification alloys. Several macrosegregation regions consisting of the curved coarsened lamellae are produced only in hypo-peritectic alloy. These regions may be induced by relatively weak thermo-gravitational convection (TGC) effect, which do not change the lamellar grain orientation but make a small orientation angle deviation of the α2 phase from each other in the grain.

Corrosion and wear performances of hydroxyapatite and boron-containing TiO2 composite coatings on Ti6Al7Nb alloy

Plasma electrolytic oxidation coupled with hydrothermal treatment is a relatively new technique to form a hydroxyapatite/TiO2 layer on titanium alloys for biomedical applications. Hence the process allows achieving a bioactive and bactericidal surface by using electrolytes that contain ions (such as calcium-phosphorus and boron) necessary for desired properties. The coating properties are controllable by adjusting the parameters in the PEO process. In the present study, an electrolyte that contains both calcium, phosphorus, and boron ions was used to form a rough and porous oxide layer on Ti6Al7Nb which is known to be less toxic than the most widely used Ti alloy for biomedical applications, Ti6Al4V. A hydroxyapatite and boron-containing oxide layer was obtained after plasma electrolytic oxidation and hydrothermal treatment. Coatings were examined by XRD, XPS, SEM, contact angle measurement system, micro-hardness tester, wear tester, and corrosion measurement system. The results showed that the wear and the corrosion properties of all coated samples increased. Especially boron doping enhanced both the wear and corrosion resistance. Relatively the best corrosion resistance was achieved from CaP-B and the best wear resistance was from HA-B samples. The hardness values and mean surface roughness of all coated samples also increased while the average friction coefficients decreased. The hardness increased from 323 ± 5 HV0.1 to 1084 ± 16 HV0.1 where the coefficient of friction decreased from 0.5672 ± 0.01 to 0.4697 ± 0.03.

Developing Microstructure and Enhancing Strength of Ti–6Al–7Nb Alloy with Heat Treatment Processed by High-Pressure Torsion

Developing Microstructure and Enhancing Strength of Ti–6Al–7Nb Alloy with Heat Treatment Processed by High-Pressure Torsion

Effects of Cooling Rate on the Microstructure and Tensile Properties of Powder Metallurgy Ti2AlNb Alloy

Effects of Cooling Rate on the Microstructure and Tensile Properties of Powder Metallurgy Ti2AlNb Alloy

Investigating the thermal oxidation behavior of Ti–6Al–7Nb alloy in dry air

AbstractThe thermal oxidation behavior of a forged Ti–6Al–7Nb (Ti67) alloy was investigated in dry air conditions. The alloy specimens were oxidized for 50 h at different temperatures (650°C, 750°C, and 850°C) and the oxidation kinetics were analyzed. The microstructure of Ti67 after oxidation and the different phases and compounds were identified. A parabolic equation based on weight gain was used to define the oxidation rate. The parabolic rate constant increased from 1.10 × 10−8at 650°C to 2.10 × 10−8and 1.53 × 10−7g2/(cm4·h) at 750°C and 850°C, respectively. At 650°C, the oxidation behavior resumed the parabolic behavior until 40 h, a breakaway phenomenon. At 750°C, a linear behavior was observed in the periods of 20–50 h. This linear trend appeared also after 35–50 h at 850°C. The activation energy was found to be 238.4 kJ/mole. At the lowest oxidation temperature (650°C), a disconnected oxide layer was observed. These variations in the response of Ti67 alloy to the different thermal oxidation conditions are worthy of consideration when modifying the surfaces for biomedical applications.

Understanding the microstructure evolution mechanism and the microstructure-strength correlations of Ti3SiC2/Ti2AlNb joint brazed with AgCu interlayer

Herein, a Ti3SiC2 ceramic was brazed to a Ti2AlNb alloy using an AgCu foil. The effect of brazing parameters on the microstructure of the brazed joint was investigated. Through careful characterizations of the interfacial reaction products, the microstructure evolution mechanism of the brazed joint was clarified. The unique crystal structure of the Ti3SiC2 phase leads to the effortless de-intercalation of Si atoms. The de-intercalated Si atoms react with Ti atoms, which dominate a series of complex interfacial reactions. The influence of brazing temperature on these interfacial reactions is strong. Maximum shear strength of 89 MPa was achieved when the joints were brazed at 860 °C for 20 min. A sound joint benefits from the detachment of Ti–Si–Cu interfacial reaction layers from the ceramic surface, the mutual diffusion zone inside the Ti3SiC2 ceramic, and moderate TiCuSi and Ti5Si3 brittle compounds in the brazed seam.

Effect of processing parameters on the microstructure and mechanical properties of TiAl/Ti2AlNb laminated composites

In order to improve the intrinsic brittleness of TiAl alloys, Ti2AlNb alloys with outstanding ductility and toughness at room temperature, and good high-temperature performance are competitive candidates in constructing the TiAl-based laminated composites. In this work, TiAl/Ti2AlNb laminated composites are successfully synthesized by vacuum hot pressing combined with the foil-foil (sheet) metallurgy. Under the pressure of 65 MPa, different holding time and temperature of hot pressing are tried and the optimized fabrication parameter is acquired as 1050 °C/120 min/65 MPa. Along with the changes of processing parameters, the defect, microstructure, interface, phase transformation and the corresponding mechanical properties are detailly discussed. The results show that the TiAl/Ti2AlNb laminated composite fabricated at 1050 °C for 2 h achieves a good metallurgical interface bonding. The corresponding interface microstructure is composed of region I and region II. The region I consists of O, α2 and B2/β phase, and region II is made up of α2. Subsequently, the tensile tests indicate that the composite synthesized at 1050 °C for 2 h possesses a maximum strength of 812 MPa and a total elongation of 1.31% at room temperature, and a strength of 539.71 MPa and the highest total elongation of 10.34% at 750 °C. The well synergistic deformation ability between the interface and the two base alloys endows the composite an excellent tensile performance. Moreover, the composite processed at 1050 °C for 2 h behaves the best fracture toughness in both arrester orientation and divider orientation with the value of 32.6 MPa.m1/2 and 30.1 MPa.m1/2, respectively. The Ti2AlNb alloy in the laminated structure effectively release the stress around the crack tip and plays a role in toughening. Further, crack deflection, crack bridging, crack blunting and fragmentation also make contributions to enhance the fracture toughness of the laminated composites.

Liquid dripping dynamics and levitation stability control of molten Ti–Al–Nb alloy within electromagnetic fields

The dripping dynamics of the electromagnetically levitated (EML) liquid Ti–Al–Nb alloy under high temperatures was investigated by both numerical simulation based on the Arbitrary Lagrangian–Eulerian method and corresponding EML experiments. A dripping formation parameter εD was defined to describe the critical shape of alloy droplet. According to the simulated results, the high-temperature dripping phenomenon took place when εD &amp;lt; 0.68, which was in good agreement with experimental data. When dripping event occurred, the Lorentz force applied on alloy droplet decreased by approximately 11.7% within 0.07 s. Three typical methods were accordingly proposed to avoid the dripping failure of a bulk liquid Ti–Al–Nb alloy, which was implemented by enhancing electric current, adjusting levitation coil diameter, or increasing coil winding number. To control the droplet shape, the deformation pattern and the flow behavior of the liquid alloy were studied in a wide current range from 700 to 1400 A. With the increase in excitation current, the cone-shaped alloy melt transformed to a rhombus, and the flow behavior transformed from a typical four toroidal flow vortexes up to a complex eight toroidal flow vortexes. Moreover, the centroid position of liquid alloy rose up significantly at first and then slowly approached to levitation ceiling.